Process for recovery of zinc



Nov 25, 1941. M, VERY 2,263,751

PROCES FOR RECOVERY OF ZINC Filed Nov. 28, 1938 2 Sheets-Sheet 1 ROASTED ZINC CONCENTRATE FLux- QKE IF I I,

ELECTRIC POWER 3 ELECTRIC do v SMELTING l- FURNACE SLAG I MIXING CHAMBER A ZINC FAN 25 I CYCLONE P- I vENT- v 24 HOLDER COOLER -/6 /9' I I f 7- FAN ' INV ENT'0R NOV. 25, 1941. V RY 2,263,751

PROCESS FOR RECOVERY OF ZINC Filed Nov. 28, 1938 2 Sheets-Sheet 2 ROASTED ZINC FUEL CONCENTRATE 5 30 RETOFIT a:

CATALYST 3 52. 1.92? A CHAMBER f J 47 .(1 (1 MIXING I l CHAMBER FAN I y Y. 5/ f5? 1 FAN CYCLONE VENT f2 50 I I 49 45 GAS 44 T GAS HOLDER COOLER l v k I SOLID L|QU|D 42 zmc zmc FAN I INVENTQR Patented Nov. 25, 1941 PROCESS FOR. RECOVERY OF ZINC Julian M. Avery, Westport, Conn., assignor to Arthur, D. Little, Incorporated, Cambridge, Mass, a corporation of Massachusetts Application November 28, 1938, Serial No. 242,672

2 Claims. (01. 75-88) This invention relates to the recovery of metallic zinc from ores by smelting processes, and more particularly to the condensation of zinc vapor from gases produced in continuous zinc smelting processes.

Zinc is usually produced by the reduction-distillation smelting of a mixture of roasted zinc ore concentrates and carbonaceous reducing agent in batteries of small externally heated batch operated retorts. It has long been recognized that this method is fundamentally uneconomical for large scale smelting operations, and much effort has been directed toward the development of continuous zinc smelting processes and apparatus adapted to simple control, large unit capacity, and highiuel economy. In most of the proposed processes the zinc must be recovered by condensation from a mixture of gases produced by the smelting reactions, whether the reduction is accomplished by means of gaseous or solid reducing agents; whether the smelting is carried out in retorts or furnaces; and whether the retorts, if used, are externally or internally heated, or both. In all such processes experience has taught that at temperatures above the boiling point of zinc. which is therefore evolved as a vapor along with carbon monoxide gas. Since zinc vapor is monatomic, substantially equal volumes of zinc vapor and carbon monoxide make up the gaseous smelt- 5 ing product, from which metallic zinc must be recovered by condensation.- cooled, the reaction (B) 2CO=CO2+C has an increasing tendency to occur, and any CO2 thus formed tends to oxidize zinc according As the gases are tothe reaction the condensation of zinc vapor from the gaseous smelting products presents problems of increasing seriousness and complexity as the volume and dilution of gases to ,be handled increases,

with the result that the development of smelting units of large capacity has been very seriously '30 hampered. The primary object of the present invention is to overcome these difilculties by providing a novel method for condensing zinc vapor from the gaseous smelting products, which is at once simple, efiective, economical and practically without limit as to capacity.

Technically one of the most promising methods for large scale continuous zinc smelting involves the use of electric smelting furnaces, but the problem of condensing zinc effectively has here- 40 tofore proved insuperable in such processes, largely.because of the formation of prohibitive amounts of blue powder (zinc oxide). It is an object of the present invention to provide means whereby the electrothermal smelting of zinc may be put on a practical basis.

The following description will make clear the difficulties which must be overcome in the condensation of zinc from gaseous smelting prods r ucts. assuming reduction by means of carbon,

for the sake of simplicity.

The reduction of zinc oxide by carbon takes place according to the reaction (C) CO2+Zn=ZnO+CO which likewise has an increasing tendency to occur as the temperature is decreased. Combining reactionsBand C:

it is apparent that as the temperature is decreased in order to condense the zinc vapor, carbon monoxide tends increasingly to oxidize metallic zinc. Thus, carbon monoxide, usually considered to be a strongly reducing gas, actually becomes in effect an oxidizing gas under the conditions required for condensation of zinc. In this manner zinc vapor is oxidized to zinc oxide and globules of liquid or solid metallic zinc produced by condensation are coated with zinc oxide. The powder formed by the accumulation of such oxide dust and oxidized zinc globules is known, from its characteristic color, as blue powder. In the old type retort process the formation of blue powder commonly amounts to from 5% to 20% of the weight of zinc produced, but incontinuous smelting processes the proportion of blue powder is usuallymu'ch greater, and cases are known in which all the zinc condensed was in the form of blue powder.

Condensation of zinc vapor to liquid zinc without fouling the condensq walls by solid accretions requires that the condenser walls be maintained at a temperature above the melting point of zinc, which results in a low rate of heat transform to the capacity of a condenser of reasonablesize. I v

The present invention is based upon the discovery that if the gaseous smelting products are.

rapidly mixed with a relatively large volume of cold recirculated zinc-free gas produced in the smelting reaction, zinc vapor can be made to condense so quickly that it is substantially removed from the gas phase before appreciable oxidation can take place according to reactions B, C, and D. Moreover, if the quantity of cold recirculated gas used is sufficient to absorb the heat of condensation of the zinc and to cool the mixture of zinc and gases to such a temperature that a major part of the zinc is condensed out of the gas phase, the resulting temperature is sufiicient- 1y low that reactions B, C and D in spite of their strong tendency to occur, proceed at such small reaction velocities that globules of metallic zinc, once formed, are not reoxidized within any reasonable time of exposure to the gas phase. In this manner zinc Vapor may be condensed out of the gaseous reaction products as solid or liquid zinc without the formation of blue powder in' deleterious amount, and without depending upon heat transfer through the condenser walls for cooling the gases, or requiring the use of an expensive reducing gas such as hydrogen derived from an outside source, as will now be shown.

In describing the procedure of the present invention, reference should be had to the accompanying drawings, wherein:

Fig. 1 represents, in flow-sheet form, a preferred series of steps in carrying out the invention, and

Fig. 2 represents, also in flow-sheet form, an alternative procedure.

The manner of operation of the present invention will be clear from the following description taken in connection with Fig. 1 of the drawings which represents schematically apreferred embodiment of the invention as applied to the condensation of zinc in solid form from the gaseous products of electrothermal smelting using recycled cooling gases. Gases passing from an electrothermal or other continuous smelting furnace l through a duct I l are mixed in a mixing chamber I2 with cold reducing gas delivered through a duct 23. The mixture of gas and zinc dust passes through a duct l3 to a cyclone dust catcher l4 from which the metallic zinc dust product is removed at 25. The gases remaining after precipitation of the zinc pass through a duct l5 to a cooler l6 and thence through a duct I! to a fan [8 which controls the movement of gas through the system. From fan l8 the gases are discharged through a duct I9 to a gas holder 20 fitted with a vent 24. Cold gas, as required, is drawn from gas holder 20 through a duct 2| by a fan 22 and delivered through duct 23 to the mixing chamber. It will be evident that the permanently gaseous products of reaction, in this case primarily CO, will accumulate in the closed cycle unless vented, and they are therefore vented through vent 24 as desired.

I have found that in such a system, assuming the gaseous reaction products to consist of approximately equal volumes of zinc vapor and carbon monoxide in accordance with reaction A, and assuming them to enter the mixing chamber at a temperature of 1400 C., an equilibrium temperature of 300 C. can be attained, and all the zinc precipitated as solid zinc, by recycling approximately kg. of cooling gas (at ambient temperature) for each kilogram of zinc condensed, or 23 volumes of recycled gas for each volume of CO produced. Assuming a smelting furnace of 5000 horse-power capacity, smelting reaction products amount to about 300 cu. ft. per min. and the recycled cooling gas required is about 6800 cu. ft. per min. These quantities of gases are readily handled in equipment of small size and the simplest possible construction.

As an alternative to this procedure, the zinc may be recovered as liquid zinc by recycling about 6 kg. of cooling gas (at ambient temperature) for each kilogram of zinc condensed. But in such case, to avoid fouling the equipment with solid accretions, the temperature must be kept so high that the liquid zinc has appreciable vapor pressure, and it may therefore be necessary subsequently further to cool the gases in any convenient manner to recover the remainder of the zinc in solid form, as further described hereinafter.

The application of the invention to a very different type of smelting process, developed by the Per cent Zn 25 CO.. 25 H2 50 The final gaseous reaction products then pass through a duct to a mixing chamber 36 where they are mixed with the desired proportion of cold recycled gas delivered through a duct 49. The gases leave-the mixing chamber through a duct 31 and enter a cyclone 38 where they are mixed with an additional quantity of cold recycled gas delivered through a duct 52. From the cyclone the gases then pass through a duct 39 to a gas cooler 40 and thence through a duct 4| to a fan 42 which maintains circulation in the system and discharges the gases through a duct 43 to a gas holder 44, which is provided with a vent 45 for voiding excess gas from the system. From the gas holder gas is withdrawn as required through ducts 46 and by fans 41 and 5| and delivered by ducts 48 and 32 to the retort, by ducts 48 and 49 to the mixing chamber, and by a duct 52 to the cyclone, in amounts as required for each.

If the gaseous reaction products are delivered I at a temperature of 1100 C., and'sufiicient gas approximately 20 tons of zinc a day, the gaseous is recycled to create an equilibrium temperature in the mixing chamber of 600 C., the amount of recycled gas required is about 10.6 cubic meters for each cubic meter of zinc vapor or each 3 cubic meters of CO+H2 in the reaction gases. At 600 C. the vapor pressure of zinc is 0.0145 at., and the dilution of gases is such that about 20% of the zinc will fail to condense under the assumed conditions. To recover the balance of the zinc I find it convenient to pass the gases to the cyclone at about 600 C., and there mix them with an additional quantity of cold recycled gas sufficient to precipitate the balance of the zinc; in solid form.

Usually some of the zinc will be caught mechanically in the mixing chamber, and if desired this chamber may be designed to serve in whole or in part the function of removing the zinc from the system. I have found it preferable to perform these two functions in separate units because of the latitude and simplicity of design so afforded, but the invention is not to be understood as limited to such a separation of function;

On the other hand, the mixing chamber and zinc collector may take any desired form and may be of any desired numbers or types, so long as the functions of (a) mixing the hot gaseous reaction products with cold recycled gas quickly to reachapproximate thermal equilibrium at a desired temperature and (b) to remove the solid or liquid zinc from the system, are accomplished.

There are of course many possible ways of bringing the gaseous reaction products and the cold recirculated gases into contact with each other and mixing them to produce thermal equilibrium, and the most desirable method must of necessity be worked out for each individual case. For example, violent mixing action or a relatively slow, diffusive type of mixing may be used, depending uponwhether large or small drops or particles of zinc are desired. Also, the flow of gases may be so controlled as to produce centrifugal separation of the condensed metallic zinc from the gases. All such variations in details of procedure are within the scope of my invention.

It is well known that in most zinc smelting processes some C02 is usually present in the initial gaseous products of smelting, and it might be anticipated that inthe closed cycle of the present invention building up of this CO2 would cause increasing formation of blue powder. I have found however that the venting from the system of a quantity of gas corresponding to the amount of zinc metal produced, as described hereinoefore, automatically keeps the concentration of CO2 in the system well under control. The vented gases, being high in fuel value, may of course be used to advantage outside the closed cycle, as for preheating the ore charge, or for other purposes.

I claim:

1. Method for the manufacture of zinc, which comprises producing a hot gaseous mixture containing essentially zinc and carbon monoxide, mixing with said gaseous mixture relatively cool recycled zinc-free gas, in sufficient volume to cause separation of a portion of the zinc from said mixture as liquid zinc, removing said liquid zinc, mixing with the resulting partially cooled zinc-containing gases a further quantity of relatively cool recycled zinc-free gas in suflicient volume to cause separation of the bulk of the remaining zinc from said gases as solid zinc, removing said solid zinc, cooling the resulting gases, and recycling said cooled gases in a closed gas cycle, a portion of said cooled gases being used for said mixing steps and the balance being vented from the system.

2. Method for the manufacture of zinc, which comprises producing a hot gaseous mixture consisting essentially of zinc vapor and a normally gaseous medium comprising carbon monoxide, mixing with said gaseous mixture relatively cool recycled substantially zinc-free gas, in suflicient volume to cause separation of a portion of the zinc from said mixture as non-gaseous zinc, re-

moving said non-gaseous zinc, mixing with the resulting partially cooled zinc-containing gases a further quantity of relatively cool recycled substantially zinc-free gas in sufiicient volume to cause separation of the bulk of the remaining zinc from said gases as non-gaseous zinc, removing said remaining non-gaseous zinc, cooling the resulting gases, and recycling a major portion of said cooled gases in a closed gas cycle.

JULIAN M. AVERY. 

